Oxalic acid is a general compound that is contained in many foods (in particular, spinach and other deep green vegetables, green tea, cocoa, etc.), or generated in a human body (generated in a process of metabolism, not decomposed further more in a living body and excreted with urine). Oxalic acid has been well known as a substance to be a factor of stone due to combining with calcium in a human body. Further, increase of an oxalic acid concentration in urine (increase of risk of stone formation) has also been observed due to excess intake of oxalic acid and an overproduction of oxalic acid in a human body. Furthermore, oxalic acid has been utilized also as an attacking means of a phytopathogenic fungus to plants. That is, there has been known existence of a phytopathogenic fungus which makes an attack such that oxalic acid is produced in infected plant tissues, thereby making the insides of the plant tissues acidic environments to blight leaves.
As described above, although oxalic acid is a general compound, it is a compound showing a harmful effect on organisms. An enzyme that decomposes oxalic acid having such an effect is useful in various fields such as the food field and the medical field. For example, oxalic acid in foods is decomposed directly or after taking in a body with an enzyme or an oxalic acid decomposing bacterium, to thus try to decrease a concentration of oxalic acid absorbed in a body. In addition, it has been performed to introduce an oxalic acid degrading enzyme into a plant body for the purpose of imparting resistance against phytopathogenic fungi.
As an enzyme that decomposes oxalic acid, an oxalate decarboxylase (hereinafter also referred to as “OXDC”), an oxalate oxidase, and an oxalyl CoA decarboxylase have been known to exist. OXDC is an enzyme that decomposes oxalic acid into formic acid and carbon dioxide, and contains manganese in its inside. It has been revealed so far that many kinds of bacteria (such as genus Bacillus) and fungi (genus Aspergillus, and Flammulina velutipes) have oxalate decarboxylase genes (hereinafter also referred to as “oxdc gene”). There have been trials to find out enzymological properties of OXDC, in particular, clarification of OXDC of Bacillus subtilis and Flammulina velutipes has progressed.
It has been reported on OXDC originating in the genus Bacillus that OXDC productivity of a Bacillus subtilis 168 strain that is a general producing bacterium is 0.4 mg/L in a culture liquid (Non-patent Document 1). An oxdc gene is cloned using Escherichia coli as a host for the purpose of clarification of enzymological properties (Non-patent Document 2). The oxdc gene recombinant Escherichia coli has higher productivity than a Bacillus subtilis 168 wild strain does, and shows OXDC productivity of 2.8 mg per 1 g of cultured bacterial bodies. However, even though this productivity is sufficient at a laboratory level, it cannot be recognized as being sufficient at a practical and commercial level.    [Non-patent Document 1] J. Bacteriol. 182 (2000), 5271-5273    [Non-patent Document 2] J. Biol. Chem. 276 (2001), 43627-43634